This invention relates to a medical diagnostic ultrasound transducer system and method. In particular, a transducer system providing increased bandwidth for imaging with different frequencies is provided.
Acoustic energy is transmitted into a patient at fundamental transmit frequencies. Acoustic energy is reflected off of tissue, fluid or other structures within the patient. The reflections include energy at the fundamental frequency band as well as energy generated at harmonic frequencies of the fundamental frequency band. The transducer converts the acoustic energy into an electrical signal.
Transducer bandwidth may limit the actual response, reducing the information content at harmonic or other frequencies. Manufacturing transducers with a 6 dB bandwidth or better exceeding 80% of the desired frequency range is difficult and expensive. For harmonic imaging, a 6 dB bandwidth exceeding 100 or 140% is preferably provided. For example, the transducer may transmit energy in a 3 to 5 MHz frequency range and receive information of interest in the 6 to 9 MHz range.
Larger bandwidth transducers are generally desirable for any type of ultrasonic diagnostic imaging. For example, transducers with a wide bandwidth are used for obtaining information at different fundamental frequencies during a same or different imaging sessions. Various techniques have been suggested for providing wide bandwidth transducers. For example, T. R. Gururaja et al in xe2x80x98Medical Ultrasonics Transducers With Switchable Frequency Bands Centered about f0 and 2f0xe2x80x99, 1997 IEEE Ultrasonic Symposium, pp. 1659-1662, disclose an electrostrictive transducer element using two layers. A selected bias is applied to one layer, and a transmit waveform is applied to an electrode between the two layers for wide bandwidth transmission. As another example, J. Hossack et al. in Improving the Characteristics of A Transducer Using Multiple Piezoelectric Layers, IEEE Transactions On Ultrasonics, Ferroelectrics and Frequency Control, Vol. 40, No. 2, March 1993, disclose a two-layer piezoelectric single element transducer. A different waveform is applied to each of the layers on transmit, and phasing or delays are applied to signals from one of the layers relative to another layer on receive. As another example, different materials in a single layer transducer element may be used to extend the frequency range of the transducer.
U.S. Pat. No. 5,957,851, the disclosure of which is incorporated herein by reference, discloses an ultrasound transducer with multiple piezoelectric layers for use in harmonic imaging. Diodes or a transistor is used to isolate one layer from the other during transmit or receive. The same transducer is used to transmit at a fundamental frequency and receive at a harmonic frequency. For this passive switching system, the same transmit and receive processing is performed for each layer when each layer is being used.
Transducer bandwidth is also important for different types of imaging modes, such as Doppler and B-mode imaging. For B-mode imaging, the highest frequency possible is used for tissue close to the surface and lower frequencies are used for tissue deeper or further away from the transducer. A dual frequency transducer covers a much wider range of frequencies than previously possible using conventional transducer designs. For example, a dual frequency transducer covers a frequency spectrum with an effective xe2x88x926 db bandwidth of 120%-130% of the operating center frequency. However, the conventional transducer designs can only obtain 75%-95% bandwidth. For the color flow and Doppler modes of imaging, the transducer can operate over a much wider frequency band to result in better signal penetration or a wider frequency band for the detection of the Doppler frequency shift.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and transducer system for selectable frequency imaging. At least one transducer element is provided. The transducer element comprises two stacked piezoelectric layers. The layers are stacked in the height or thickness direction which is perpendicular to an elevation-azimuth plane (i.e. azimuth is X direction, elevation is Y direction and range is Z direction). Information from each of the layers is independently processed during one of a transmit event, a receive event, and both of transmit and receive events by applying relative delays and/or amplitude changes between the layers. The frequency response of the bi-layer transducer element is adjusted as a function of the delay and amplitude control, controlling the operation frequency and/or increasing the impulse-echo bandwidth or effective bandwidth of the transducer. The frequency response of the bi-layer transducer element is modified by interference so that the signals for each layer constructively interfere in the frequency region of the desired peak response. Alternatively, destructive interference is created in a frequency band to be suppressed.
In a first aspect, each of the layers of the transducer element comprise a material with different acoustic properties and transfer characteristic, such as one layer of solid ceramic material and the other layer of piezo-composite material. Using relative delay and/or amplitude adjustments, the different transmit or receive processing accounts for the different acoustic transfer characteristics, providing selectable frequency appropriate interference.
In a second aspect, a matching layer of the transducer element varies as a function of elevation across the element. For example, varying the thickness or grading provides for lower frequency operation at the edges of the elevation aperture and higher frequency operation at the center of the elevation aperture.
In a third aspect, a multiplexer selectively operates the transducer element in different resonance frequency modes. For example, a single transmit and/or receive system channels selectively connect with one or more of electrodes between layers, on top of the element or below the element. The transducer element is operated for receiving at the same frequency as the transmit frequency or receiving at a different frequency than the transmit frequency.
Further aspects and advantages of the invention are described below in conjunction with the preferred embodiments.